1. Field of the Invention
The present invention relates to a color plasma display panel used for an information display terminal, a flat panel TV receiver or the like, and, more particularly, to a panel structure for attaining high contrast, high brightness and high efficiency for light emission.
2. Description of the Related Art
A color plasma display panel is a display which excites phosphor with ultraviolet rays generated by gas discharge, thereby causing the phosphor to emit light for display. It can be classified into an AC type and a DC type depending on the form of discharge. Of them, the AC type is superior to the DC type with its brightness, luminous efficiency, and life. Of the AC type, a direct view type AC surface discharging type is superior with its brightness and luminous efficiency.
FIG. 14 is a sectional view of an example of a conventional direct view type AC surface discharging color plasma display panel. A transparent electrode 2 is formed on a front substrate 1 which is a transparent glass plate constituting the display surface. The transparent electrode is formed in a plurality of stripes in a direction parallel to the surface of paper sheet. Pulse AC voltage from several tens to several hundreds KHz is applied between the adjacent transparent electrodes 2 to obtain display discharge.
Tin oxide (SnO.sub.2) or indium tin oxide (ITO) is used for the transparent electrode 2. Employed to lower resistance is an electrode provided therealong a bus electrode made of a multilayer thin film of chromium/copper/chromium, a metal thin film such as an aluminum thin film, or a metal thick film of silver or the like. When it is formed by a silver thick film, slight amount of black pigment is often mixed. However, the bus electrode is omitted in FIG. 14.
The transparent electrode 2 is coated with a transparent insulating layer 17. The transparent insulating layer 17 has a function for limiting current which is unique to the AC type plasma display. In view of dielectric breakdown voltage or ease of manufacturing, the transparent insulating layer 17 is typically formed by applying paste containing low melting point lead glass, firing and reflowing it at a raised temperature higher than its softening point. This provides a flat transparent insulating layer 17 not containing an air bubble therein and with a thickness of about 20 microns to 40 microns. A black matrix layer 30 is formed thereon. This serves to reduce reflection of external light on the display surface, and has an effect of reducing erroneous discharge and optical crosstalk between adjacent discharge cells. The black matrix 30 is also typically formed by applying paste consisting of metal oxide powder such as chromium or nickel and low melting point lead glass with thick film printing.
Then, a protective layer 16 is formed to coat the entire structure of the transparent insulating layer 17 and the black matrix layer 30. It is a thin film of MgO formed by vapor deposition or spattering, or a thick film of MgO formed by printing or spraying. It has a thickness of about 0.5 microns to 2 microns. The protective layer serves to reduce the discharge voltage and to prevent surface spattering.
On the other hand, formed on a rear substrate 8 which is a glass plate is a data electrode 9 for writing display data. In FIG. 14, the data electrode 9 extends in a direction perpendicular to the sheet surface, and formed for each of discharge cells 18-20. That is, the data electrode 9 is orthogonal to the transparent electrode 2 formed on the front substrate 1 which is a glass plate. The data electrode 9 is coated with a white insulating layer 7 which is formed by printing and firing thick film paste, comprising a mixture of low melting point lead glass and white pigments. Typically, titanium oxide powder or alumina powder is used as the white pigment. A white partition 6 is typically formed on the white insulating layer 7 through thick film printing or sand blasting. Then, phosphor (red) 10, phosphor (green) 11, and phosphor (blue) 12 are applied on the discharge cells 18, 19 and 20, respectively. Each phosphor is also applied on sides of the white partition 6 to increase the area on which the phosphor is applied and to obtain high brightness. Typically, screen printing is used for formation of each phosphor film.
The front substrate 1 is lined and air-tightly sealed to the rear substrate 8 so that the pattern of black matrix layer 30 formed on the front substrate 1 overlaps the white partition 6 formed on the rear substrate 8, Dischargeable gas such as a mixture of He, Ne and Xe is sealed in each discharge cell 18-20 under a pressure of about 500 torr.
In FIG. 14, each of discharge cells 18-20 is arranged with two transparent electrodes 2 between which surface discharge occurs to produce plasmas in the discharge cell (red) 18, discharge cell (green) 19 and discharge cell (blue) 20. Ultraviolet rays generated at that moment excite the phosphor (red) 10, phosphor (green) 11 and phosphor (blue) 12, causes them to emit visible light, thereby obtaining emission for display through the front substrate 1.
A set of adjacent transparent electrodes 2 generating surface discharge serves as a scan electrode and a sustain electrode, respectively. In actually driving the panel, a sustain pulse is applied between the scan electrode and the sustain electrode. When write discharge is to be generated, opposing discharge is generated by applying a voltage between the scan electrode and the data electrode 9. Such discharge is maintained by the sustain pulse subsequently applied to write pulse between the surface discharge electrodes.
FIG. 5 shows another conventional example. It is a one in which the black matrix 30 of FIG. 14 is increased for its film thickness to form a black partition 5. The basic process is same as that in FIG. 14. The black partition 5 is typically formed with screen printing or sand blasting. Materials used are low melting point lead glass, a filler material such as alumina, and a black pigment. The black pigment used is a one similar to that for the black matrix 30. This structure has a smaller area of applied phosphor than that of the structure of FIG. 14 so that its brightness is slightly reduced. However, since some distance can be maintained for the phosphor at the top of the white partition 6 from the surface discharge generated along the front substrate 1, there is an advantage that there is small variation in brightness even after lighting for a prolonged period of time.
The phosphor used for the color plasma display panel is white powder with very high reflectance. In the conventional color plasma display panel as described for FIG. 14 or 15, when light in a room or outdoor (external light) is incident on the panel, the external light is absorbed by the black matrix or black partition, or the bus electrode, but about 30%-50% is reflected so that contrast or color purity is significantly degraded. Thus, while there is an approach to arrange an ND filter with transmissivity of about 40-80% on the panel surface, it has a disadvantage that brightness of the panel is reduced because it also absorbs the emission from the panel.
There is an approach to use micro-color filters so that the panel brightness is not reduced as possible, and the reflection of external light is reduced. This is an approach to provide color filters for transmitting red, green and blue light on the display surface in correspondence to color emitted from respective discharge cells of red, green and blue. The micro-color filter for the plasma display is formed by a method for directly forming it on the surface of glass substrate, or a method for constituting the insulating layer of the AC plasma display with a tinted glass layer. FIG. 15 shows a sectional view of an example of conventional color plasma display panel using the latter method. This forms color filters transmitting color emitted from a discharge cell (red) 18, a discharge cell (green) 19 and a discharge cell (blue) 20 on the transparent electrode 2. The structural difference from FIG. 14 lies in that the transparent insulating layers 17 coating the discharge electrodes are replaced by a color filter (red) 13, a color filter (green) 14 and a color filter (blue) 15 which are constituted by tinted low melting point lead glass layers. This structure is known from Japanese Laid-Open Patent Publication Hei. 4-36930. This enables it to suppress attenuation of light emitted from each discharge cell 18-20 at the minimum level, to suppress reflection of external light, and to improve contrast.
Each of the color filters 13-15 is formed as an insulating layer of tinted low melting point lead glass generally by mixing low melting point lead glass powder and pigment powder and printing filter paste which is mixture of organic solvent and binder for each color with screen printing, and firing it. Here, since pigment powder should withstand against the firing process at a high temperature (500.degree. C.-600.degree. C.), inorganic materials are selected. Typical pigment powder is:
Red: Fe.sub.2 O.sub.3 type PA1 Green: CoO--Al.sub.2 O.sub.3 --TiO.sub.2 --Cr.sub.2 O.sub.3 type PA1 Blue: CoO--Al.sub.2 O.sub.3 type,
The filter paste is separately printed in three passes for each color of red, green and blue to form the entire color filter layer.
FIG. 6 shows a case where, in a similar structure, a black partition 5 is formed instead of the black matrix 30.
The color filter layer is necessary to have a thickness of 20 microns or more so that it can also serve as an insulating layer with sufficient dielectric breakdown voltage. This causes a recess or raise at the joint of each color of the color filters. It adversely affects dielectric breakdown or the post process for black matrix or black partition.
To avoid such adverse effect, known from Japanese Laid-Open Patent Publication Hei. 7-21924 is a method which flattens the entire surface of the color filter by further coating the low melting point tinted lead glass color filters 13-15 with a transparent insulating layer 4 as shown in FIG. 7. In addition, to attain the structure of FIG. 15 or 6, Japanese Laid-Open Patent Publication Hei. 4-249032 discloses a method in which low melting point lead glass paste is applied on the entire surface after each color pigment is separately painted and arranged for diffusing and dispersing the pigment in the low melting point lead glass layer.
The conventional color filter layer constituted by dispersing pigment powder in the low melting point lead glass causes scattering of light because the refraction index differs between the pigment and, the low melting point lead glass. This leads to a disadvantage in that parallel ray transmittance is deteriorated for the filter. Here, the parallel ray transmittance means transmittance of light substantially linearly transmitting through the filter, and does not include components of light scattered by the filter. As such, since the color filter has high scattering characteristics, the external light is back scattered, which deteriorates the effect as the color filter. That is, it causes opaque screen display. In addition, since the color of the color filter itself is further emphasized, there is a disadvantage that a feeling of disorder occurs particularly in displaying black. In addition, there is a problem that the color emitted from the discharge cell is reduced to lower brightness. Furthermore, the pigment is not often uniformly dispersed in the low melting point lead glass film, but aggregated therein, so that the performance as the color filter may be extremely degraded. Moreover, when a pigment is dispersed in the low melting point lead glass, there may be a problem that the pigment suffers from discoloring or change of color.
Furthermore, thorough experiments revealed that the pigment might cause the transparent electrode consisting of an ITO or Nesa (SnO.sub.2) film to react with the pigment when fired at a high temperature so that the performance of color filter may be deteriorated. For example, for a CoO--Al.sub.2 O.sub.3 type pigment which is excellent as a blue pigment, there is a problem that light is absorbed near a wavelength of 400 nm through the firing process to significantly reduce transmittance as the blue filter, thereby causing reduction of panel brightness and destruction of color balance. In addition, the red filter paste using an Fe.sub.2 O.sub.3 type pigment has a problem that significant discoloring is caused by reaction with the transparent electrode, whereby the function of color filter is impaired. Such phenomenon are not clear whether they are caused from direct reaction or catalysis of the transparent electrode material, but problems to be solved to realize good color filters.
In addition to the deterioration of color filter performance as described above, since the arrangement for dispersing the color pigment in the low melting point lead glass accompanies reflow due to firing, there arise such a problem that the fine color filter pattern is offset or spread into an area surrounding a predetermined pixel.
These problems prevent a color plasma display panel with excellent display performance having good color filters from being put in practical use.